Diode-pumped lasers having solid-state gain media incorporating ytterbium ions (Yb3+), neodymium ions (Nd3+), or titanium ions (Ti3+) generate laser radiation at near-infrared wavelengths with very high efficiencies. This near-infrared laser radiation is often referred to as “fundamental” laser radiation by practitioners of the laser art. Lasers are commercially available for generating fundamental laser radiation over a broad parameter space. Industrial fiber lasers having Yb3+ doped optical fibers in serial arrangements can produce beams of continuous wave laser radiation at a wavelength of about 1 micrometer (μm) and at kilowatt average powers. Industrial ultrafast lasers having Yb3+ doped optical fibers can produce pulses of laser radiation at megahertz (MHz) repetition rates and with compressed pulse durations from hundreds of femtoseconds (fs) to tens of picoseconds (ps). Ultrafast lasers having Ti3+ doped sapphire slabs in multi-pass arrangements can produce pulses of laser radiation at wavelengths of about 800 nanometers (nm) and at pulse energies exceeding 1 millijoule, with compressed pulse durations of tens of femtoseconds.
Nonlinear optical processes in nonlinear optical materials are used to convert fundamental radiation to other wavelengths that are useful in particular applications. Some wavelengths are accessed using cascaded nonlinear optical processes in a series of nonlinear materials. In harmonic generation processes, new wavelengths are created corresponding to frequencies that are harmonics of the frequency ωF of the fundamental laser beam. For example, in a second-harmonic generation (SHG) process, pairs of photons in a fundamental laser beam are converted into photons in a second-harmonic beam having a frequency ωSH=2ωF. In an optical parametric amplification (OPA) process, a “pump” laser beam having a frequency ωp is converted into a “signal” beam having a frequency ωS and an “idler” beam having another frequency ωI, with ωp=ωS+ωI to conserve energy. Although optical parametric amplification can begin spontaneously, the overall efficiency can be improved by injecting a low-power “seed” beam having the signal frequency ωS into the nonlinear material, which is amplified by the OPA process. The optical parametric amplification process transfers power from the pump beam to the seed beam, thereby amplifying the seed beam, which becomes the signal beam. Usually, nonlinear optical materials in commercial lasers are birefringent crystals. Momentum conservation is achieved by careful selection of the polarizations and propagation directions of the interacting beams with respect to the crystallographic axes. This selection is known as “phase matching” by practitioners of the art.
Optical parametric amplification processes using a fundamental beam as the pump beam (ωP=ωF) are used to generate signal and idler beams having desired infrared wavelengths. To generate beams having visible or near-infrared wavelengths, a cascaded process having two nonlinear stages may be used. In a first stage, second-harmonic generation in a first nonlinear crystal generates a second-harmonic beam. In a second stage, optical parametric amplification in a second nonlinear crystal uses the second-harmonic beam as the pump beam (ωP=ωSH). A seed beam is also injected into the second nonlinear crystal with a frequency selected to produce signal and idler beams having the desired visible or near-infrared wavelengths. Ultrashort pulses from ultrafast lasers have high-peak powers for efficient nonlinear conversion in SHG and OPA processes.
FIG. 1 schematically illustrates an example of such a prior-art apparatus 10 for generating a visible or near-infrared beam of laser radiation. A fundamental laser beam 12 is converted into a second-harmonic laser beam 16 in a first nonlinear crystal 14 by second-harmonic generation. Second-harmonic beam 16 and a seed laser beam 18 are directed into a second nonlinear crystal 20, wherein seed beam 18 is amplified by optical parametric amplification. A signal beam 22 and an idler beam 24 are thereby generated. Seed beam 18 has the signal beam frequency φs. Sources of fundamental beam 12 and seed beam 18 are not depicted for convenience of illustration. Orientations of the crystallographic axes with respect to the surfaces of the nonlinear crystals and the angles-of-incidence of the respective beams on those surfaces are selected to phase match the two nonlinear processes. It should be noted, in general, that an OPA process requires a non-collinear arrangement of beams for phase matching. Non-collinear angles between beams inside a crystal correspond to larger non-collinear angles outside the crystal due to refraction.
By way of example, a fundamental beam having a spectrum spanning a wavelength range 1020-1080 nm, produced by an ultrafast laser with a Nd3+ doped glass as the gain medium, is converted into a green second-harmonic beam spanning a range 510-540 nm. A seed beam having a red wavelength of 700 nm is then amplified, producing an infrared idler beam spanning a range 1880-2360 nm. Either the amplified red seed beam or the infrared idler beam could be used in an application of apparatus 10.
While pumping second crystal 20 by a second-harmonic beam allows for broad spectral tuning range, the overall efficiency of the SHG process in first crystal 14 is limited by back conversion, which is the conversion of second-harmonic radiation back into fundamental radiation by the same nonlinear process. Back conversion clamps the overall SHG efficiency at about 50% and thereby reduces the efficiency of apparatus 10 by about a factor of two.
There is need for less-complex and less-expensive apparatus for generating laser radiation in the visible and the infrared regions of the electromagnetic spectrum. Preferably, this apparatus would generate this laser radiation with higher efficiency than current apparatus.